Here is a silly slide that compares the improvements in CPU speeds (red) with GPU speeds (green). The point I want to make with this slide is that GPU speed improvements are on a steeper trajectory than CPUs. What happens when these curves meet, I can't say. Will GPUs overcome CPUs, or will the curves merge?
Greg Humphreys argued here that if you compared the CPU with GPU in terms of capabilities, then the GPU has already surpassed the CPU.